1. Field of the Invention
The invention relates to an image forming apparatus such as a composite machine which bears the copying function and the printing function of a copying machine and a printer, and an optical multi-beam scanning device which is mounted on the image forming device. Specifically, the invention relates to the optical multi-beam scanning device for writing latent images using a plurality of light beams on one photosensitive drum, and the image forming apparatus.
2. Description of the Related Art
In the case where a surface to be scanned on a photosensitive drum is scanned by an optical scanning device, when a light beam vertically enters the photosensitive drum, a part of the light beam which enters the photosensitive drum reflects so as to return to a light deflecting device. The reflected light beam is reflected as a secondary reflected light so as to return to the surface to be scanned again become a fixed stray light. For this reason, in general, a light beam is allowed to enter the photosensitive drum at an angle which tilts with respect to a vertical scanning direction. In this case, one light beam does not arise a problem, but when latent images are written by a plurality of light beams, a distance from a deflector to an image surface varies with the light beams. For this reason, a value “f” of an fθ characteristic varies with the light beams, and when an image is written with one image frequency, a position in a horizontal scanning direction shifts.
In a circle A of FIG. 18, conventional emitting positions of two light beams LBa and LBb in the vertical scanning direction are enlarged, and in a circle B of FIG. 18, conventional emitting positions of the two light beams LBa and LBb at one scanning end of the horizontal scanning direction are enlarged. Since an optical path from the light beam LBa to a surface to be scanned (the surface of photosensitive drum) SUR is shorter than that from the light beam LBb to the surface to be scanned, a displacement ΔH takes place between the light beams LBa and LBb even in the horizontal scanning direction at the same deflection angle according to a difference ΔD of the optical path lengths as shown in the circle B of FIG. 18.
Such a method of solving the displacement in the horizontal scanning direction is disclosed in U.S. Patent Publication No. 2003/0043441 A1.
In the method disclosed in this publication, different wavelengths are applied to two light beams, and a difference in magnification due to a difference of optical paths is canceled by magnification chromatic aberration of a post-deflection optical imaging system so that displacement is removed. A horizontal synchronized signal is input directly into a sensor without through the post-deflection optical imaging system, and when the beams come to the same position on the surface to be scanned, they are detected.
In the method disclosed in the publication, however, write timing of an image is determined based on an output of a horizontal synchronization sensor for detecting light beams which do not go through the post-deflection optical imaging system. For this reason, the following problems (1) to (3) arise.
(1) In order to allow the light beams which do not go through the post-deflection optical imaging system to enter the sensor for detecting light beams to produce the horizontal synchronized signal, it is necessary to secure a sufficient distance in the horizontal scanning direction from a light beam which goes through the post-deflection optical imaging system so as to be imaged on an image effective area to a light beam for obtaining horizontal synchronization (since an edge portion of the optical system cannot be used, the light beam for horizontal synchronization should be made to pass through the outside of the edge portion). As a result, it is necessary to enlarge a size of a polygon mirror or decrease a number of surfaces of the polygon mirror. When the size of the polygon mirror is increased, windage increases, and heat generation and noise become large. On the other hand, when a number of the surfaces of the polygon mirror is decreased, it is necessary to heighten a revolution speed in order to cope with a uniform process speed. As a result, windage increases, and heat generation and noise become large.
(2) An arrangement of optical elements in the post-deflection optical imaging system or a principal ray emitted from a pre-deflection optical system occasionally deviates from a design value. In this case, a time difference that a plurality of beams reaches a predetermined position of the horizontal synchronization sensor occasionally shifts from a time difference that the beams reach a predetermined position on the surface to be scanned.
(3) A relative position relationship among the beams on the surface to be scanned is not maintained on the surface of the horizontal synchronization sensor to which a plurality of beams which do not go through the post-deflection optical imaging system are given. That is to say, the horizontal synchronization sensor cannot be provided with a function for detecting the beam relative position and cannot make a control using an actuator based on the information about the beam relative position.